1. Field of the Invention
The present invention relates to a method and apparatus for predicting the amount of deformation due to shrinkage of a molded article, more particularly relates to a method and apparatus for predicting the amount of deformation due to shrinkage of a molded article having a base on which a rib is provided.
2. Description of the Related Art
Plastic parts etc. formed by injection molding have long been extensively used. Improvement of the dimensional precision of plastic parts etc. is being sought in order to meet the demands for improvement of the dimensional precision of auto products etc. in which these plastic parts are being used.
Such an injection molded article in general shrinks and deforms when the molten state molding material solidifies. This deformation is mainly due to shrinkage in a direction perpendicular to the flow direction of the plastic or other molding material in the mold at the time of injection. Further, when the injection molded article is plate shaped, the deformation is due more to the shrinkage rate in the thickness direction than the shrinkage rate in the planar direction. That is, anisotropy of the shrinkage rate occurs. Further, an injection molded article is sometimes made to include talc, glass fiber, or another filler having anisotropy so as to improve the strength or add special functions. It is known that if such a filler having anisotropy is included, the anisotropy of the shrinkage rate further increases.
In plastic parts etc. formed by such injection molding, that shrinkage becomes a cause lowering the dimensional precision.
For example, Japanese Patent Publication (A) No. 2004-9511 and Japanese Patent Publication (A) No. 9-311114 disclose a method of predicting the amount of deformation of an injection molded article for feedback to the product design of the injection molded article etc. and improvement of the product quality and reduction of the development costs.
Japanese Patent Publication (A) No. 2004-9511 discloses a method of predicting warping deformation of a corner of a molded article due to the temperature change accompanying cooling at the time of injection molding which sets the shrinkage rate in the thickness direction of the corner to a value larger than the shrinkage rate in the surface direction so as to predict the warping direction of the corner.
Japanese Patent Publication (A) No. 2004-9511 discloses a method of predicting a slant angle dθ when an injection molded article 10 provided with a corner 16 having an original shape of a cross-section as shown in FIG. 1A (shape of mold cavity) freely deforms as shown in FIG. 1B due to shrinkage at the time of cooling after injection of the material into the mold. This dθ is the amount of reduction of the apex angle due to deformation of the corner. Expression by the following formula (1) is disclosed in Japanese Patent Publication (A) No. 2004-9511.
                              d          ⁢                                          ⁢          θ                =                              (                          π              -              α                        )                    ⁢                                    (                                                ɛ                  p                                -                                  ɛ                  t                                            )                                      1              -                              ɛ                t                                                                        (        1        )            
Here, α indicates an apex angle of a corner before deformation, εp indicates a shrinkage rate of an injection molded article 10 in the planar direction, and εt indicates a shrinkage rate of an injection molded article 10 in the thickness direction.
FIG. 1B shows the state where the injection molded article 10 freely deforms. In FIG. 1B, the shape of the deformed injection molded article 10 is shown by the solid line, while the shape before deformation is shown by the broken line. In FIG. 1B, the corner 16 is drawn with a gap of dθ, but in actuality such a gap does not always occur. In the state of free deformation of the injection molded article 10, usually a state where the apex angle of the corner deforms to become α-dθ is obtained. Here, to clearly show the slant angle dθ, the drawing shown in FIG. 1B is used. In a later drawing for explaining the slant angle in the state of free deformation of the injection molded article, a similar drawing is used again.
Further, due to the rigidity of the injection molded article 10, the affixing of the corner 16, or other reasons, the injection molded article does not always freely deform as explained above. In this case, the corner 16 of the injection molded article 16, as shown in FIG. 1C, is acted on by a bending moment M due to shrinkage of the molding material and experiences internal stress trying to cause the corner 16 to deform.
This bending moment M can be found in the following way. If considering the geometric model shown in FIG. 1C from the state of free deformation shown in FIG. 1B, the relationship of dθ and the bending moment M is given by the following formula (2). This formula (2) can be derived by a known method if considering the free deformation of the corner 16 to be the case where a rectangular cross-section is bent to a cylindrical shape and considering that the radius of curvature of the cylinder is sufficiently large compared with the thickness H after shrinkage of the injection molded article 16 and the slant angle dθ is sufficiently small.
                    M        =                                            EH              3                                      12              ⁢                              (                                  1                  -                                      v                    2                                                  )                                              ⁢          tan          ⁢                                          ⁢          d          ⁢                                          ⁢          θ          ⁢                                          ⁢                      1            L                                              (        2        )            
Here, E indicates a Young's modulus of the molding material, ν indicates a Poisson ratio of the molding material, and L is a length between the support point and the force point of the bending moment M, specifically the thickness after shrinkage. The support point P of the bending moment M becomes the apex of the corner 16.
Further, injection molded article 10, as shown in FIG. 2A, sometimes has a base 12 on which a rib 11 is provided. When such an injection molded article 10 freely deforms due to shrinkage, as shown in FIG. 2B, this rib 11 also deforms in the same way as the above-mentioned corner 16.
Therefore, to predict the amount of deformation of the injection molded article 10 such as shown in FIG. 2A, the amount of deformation of the rib 11 is predicted using the same technique as explained above while considering the support point P of the bending moment M to be the position shown in FIG. 2B. In FIG. 2B, the shape of the deformed injection molded article 10 is shown by the solid line, while the shape before deformation is shown by the broken line. However, the predicted amount of deformation became a value larger than the actual amount of deformation of the rib 11. An accurate value could not be predicted. For that reason, there was the problem that in an injection molded article 10 having a base 12 on which a rib 11 was provided, the dimensions of the rib could not be accurately predicted.
Further, Japanese Patent Publication (A) No. 9-311114 discloses a method of predicting a PVT characteristic of a crystalline material in a non-heat equilibrium state of a crystalline material.
In general, the deformation caused by shrinkage of an injection molded article can be roughly classified into deformation due to shrinkage in solidification from a molten state liquid phase to solid phase, deformation due to shrinkage due to further crystallization of the solidified molding material, and deformation due to heat shrinkage accompanying a change in temperature of a solid phase from a high temperature state to a low temperature state. This shrinkage changes depending on the fluid state of the molding material or the temperature distribution at the time of deformation, so usually differs depending on the position inside the injection molded article. Further, if talc, glass fiber, or another filler having anisotropy is added to and dispersed in the molding material, due to that anisotropy, the anisotropy of the shrinkage rate of the injection molded article is enhanced. In this way, an injection molded article has a distribution of shrinkage rates.
However, the method of predicting the PVT characteristic described in Japanese Patent Publication (A) No. 9-311114 is not designed for the case where the molding material is comprised of a plastic and filler. The method of predicting the PVT characteristic of Japanese Patent Publication (A) No. 9-311114 precisely finds the PVT characteristic of the plastic part of the molding material, but does not precisely predict the distribution of shrinkage rates of an injection molded article to which a filler having anisotropy is added.
Further, Japanese Patent Publication (A) No. 9-311114 finds the crystallization degree in the non-heat equilibrium state by thermofluid field analysis, so is a time-consuming, troublesome technique and is not suited to the efficient design and production of an injection molded article.